“Heidi Murkoff, author of the world’s best selling pregnancy and parenting series, What to Expect, tells us about development at 4 weeks pregnant.
Your baby is smaller than the size of a seed, and the growth of the embryo and amniotic sac are already happening!
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Heidi Murkoff: It’s implantation time! Now no bigger than a poppy seed, though a whole lot sweeter, the ball of cells that will become your baby has completed its weeklong voyage down your fallopian tube and has claimed its berth in your uterus by tunneling into the sticky uterine lining. Nestled in your uterine home, this miniature miracle has established the cozy connection that will bind the two of you for the next eight plus months and forever after.
But that’s just the beginning of the action this week. In the works is the amniotic sac that your baby will call home sweet home. And at last! The sign you’ve been waiting for. Your little embryo starts to produce HCG, the just-for-pregnancy hormone that’ll turn your pregnancy pee test positively positive. You are officially expecting!
Though your embryo is only a tiny dot, it has already started to differentiate into three layers, each with its own job assignment. The inner layer, called the endoderm, will become the digestive system, the liver, pancreas, bladder, and thyroid gland.
The middle layer, the mesoderm, will grow into the heart, bones, kidneys, muscles, and blood vessels. The outer layer, called the ectoderm, will develop into the brain, nervous system, skin, hair, and eyes. It’s hard to believe, but by the end of your fourth week of pregnancy, all the building blocks for your baby’s vital organs are firmly in place.